1. Field of the Invention
The present invention generally relates to the art of chemical or fermentation processes and equipment therefor. More particularly, it is concerned with a vane-type impeller capable of performing a gas-liquid contact which may frequently be required in chemical or fermentation process to improve the efficiency thereof.
Fermentors equipped with impellers and aerating spargers have been customarily used in the field of fermentation industry since the introduction of so-called submerged culture in the production of penicillin.
As well known in the art, uniform dispersion of microorganisms throughout the liquid broth in the fermentor and sufficient supply of substrate to the microorganisms are fundamental requirements in an aerating and agitating operation of the fermentation process.
In the other words, continuous supply of various substrates, e.g., nutrients, oxygen and hydrogen ion into the liquid broth and removal of excreta and exhaust gas of the microorganisms therefrom must be performed under suitable admixing while avoiding undesirable localization of these substrates as well as heat.
Among the constituents of the substrates, oxygen is essential for the growth of most industrially utilizable aerobic microorganisms but it is only sparingly soluble in any aqueous medium.
2. Description of the Prior Art
As is well known, the ability of equipment for supplying the liquid medium with oxygen is dominating the entire progress of the fermentation in ones which requires a vast amount of oxygen, for instance, gluconic acid production from sugars and yeast production utilizing substrate of molasses or normal paraffines. Therefore, a large amount of power consumption for the aeration and agitation of the liquid medium has been considered to be inevitable in conventional equipment.
The cost for the consumed power for aeration and agitation may sometimes occupy over 70% of the entire utility expense. In addition to this, a major proportion of the consumed power has been converted into heat which might frequently be removed with additional expenses.
There has hitherto been known a system equipped with a sparger or a perforated pipe distributor which includes a gas injecting means wherein gas is injected through the axis of rotation of an impeller for the purpose of providing fine bubble distribution in the liquid medium.
The former system employs a sintered metal or ceramic filter as the diffuser solely or combined with a blade-type impeller. The high oxygen diffusing ability of the stated spargers observed with a system of water and an inorganic solute, for instance, sodium sulfate is reduced conspicuously with a fermentation broth whose physical characteristics are utterly different from those of the former system.
In addition to this, no significant reduction in the power consumption is observed with a combination of paddle-type impeller with a perforated pipe distributor as compared with another combination of impeller with a single straight-type nozzle. Moreover, possible clogging of the perforations or interstices between and among the particles of the sintered body with clots of the microorganisms from the fermentation broth might make a lengthy and continuous operation or repeated operations very difficult.
As can be easily seen, the latter system of inletting gas through a rotating axis of an impeller has no distinct advantage over the former in view points of design, operation and maintenance.
An excessive accelation of liquid flow usually accompanied by mixing operation might damage the miroorganisms in some instances and lead to the generation of useless frictional heat and wasteful consumption of energy resources.
Hence, the advent of an efficient aeration system has been eagerly desired though, a practical one which is satisfactory in and all respects has not yet been proposed to date.